Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.429s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 429
Other
Accretion, Aluminum-26 Heating, Bovedy, Chondrites, Chondrules, Impact Melting, Ste. Marguerite
Scientific paper
Planetary models for chondrule formation are not widely favored [1]. During the past decade, however, the importance of planetary impacts in the early solar system has featured prominantly in the fashionable "single large impactor" hypothesis for the origin of the Moon. If the Moon was, indeed, born out of the rapidly re-accreted debris of a planetary collision, then it may be supposed that other, smaller bodies (such as the chondrite parent bodies) could have been formed in a similar way, albeit on a more modest scale. The recent discovery in Bovedy (L3) of an immiscible glassy chondrule that formed from a silica pyroxenite precursor (a fractionated planetary rock) argues strongly in favor of this concept [2]. When the Bovedy evidence is placed alongside the considerations listed below, a general case for chondrule formation by early planetary collision seems attractive. 1. Radiogenic 26Mg began to accumulate in Ste. Marguerite about 5 Ma after it was first trapped in Allende CAIs [3]. Assuming uniform distribution of 26Al, planetesimals of 100 km radius or more formed during the first 2 Ma of this 5- Ma interval would have overheated and melted [4]. Collisions between such bodies would have discharged showers of molten silicate and metal into space, some of it escaping from the solar system altogether, some of it re-accreting sooner or later into new planetesimals (or onto existing ones) where it would again become ingested as melt. Through time, planetesimals would become fewer and larger, and their mutual collisions more energetic. Between about 4 and 5 Ma, with 26Al radioactivity now mostly spent, the bodies would have begun to cool and fractionate into a carapace of volcanic igneous rock (mixed with heterogeneous ejecta) overlying residual silicate melt, crystal cumulates, and molten metal/sulphide. Collision at this stage would yield droplets and rock fragments broadly comparable to those seen in chondritic meteorites. The accreted debris would be insufficiently radioactive to melt. 2. Alternative models involving remelting of dust in the nebula [1] require the fortuitous aggregation of precursor material into discrete (e.g., olivine-rich, pyroxene-rich, and metal-rich) compositional clusters before melting. 3. Collisions would produce enormous numbers of incandescent droplets simultaneously, sufficient to retard radiative heat loss and yield the relatively slow cooling rates implied by chondrule textures [5]. Also, volatile elements like Na may have remained close at hand as vapor, and later recondensed onto the cooling chondrule surfaces [6]. 4. Prior to collision, each planetesimal would have been surrounded by an orbiting torus of infalling debris, including primary interstellar dust. Mixing of the incandescent spray with the orbiting dust could explain a number of chondritic features, including accretionary chondrule rims, a source of nuclei to initiate crystallization, and the presence of unheated interstellar dust in the matrix of some meteorites. Also, re-integration of the disrupted planetesimal materials and intermingling of interstellar dust and other debris would help to maintain the primitive chemistry of chondrites and to provide an appropriate variety of chondrules and clasts. 5. The presence of more than one example of a very unusual and distinctive kind of chondrule in a particular meteorite (e.g., silica pyroxenite in Bovedy) suggests that accretion occurred near to the chondrule source, and was probably therefore rapid. In this case, hot accretion rather than metamorphic reheating is a plausible scenario. Besides, heating by 26Al decay is only possible during a short and specific time interval [4]. References: [1] Grossman J. N. (1988) in Meteorites and the Early Solar Sytem (J. F. Kerridge and M. S. Matthews, eds.), 680-696, Univ. of Arizona. [2] Hill H. G. M., this volume. [3] Zinner E. and Gopel C. (1992) Meteoritics, 27, 311. [4] Morden S. J. (1992) Meteoritics, 27, 263-264. [4] Radomsky P. M. and Hewins R. H. (1990) GCA, 54, 3475-3490. [6] Matsunami S. et al. (1992) Meteoritics, 27, 256-257.
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